I created a custom master page for SharePoint 2013 and had the same issue. I was also missing the Site Settings, Share, Follow and Focus On Content icons. Like you, the resolution was in the CSS. I have a pretty extensive CSS Reset file and when I removed the CSS Link everything returned to normal. In digging a little deeper here is the exact CSS style I needed to remove.

img { height: auto; max-width: 100%; }

I had this exact problem and it was caused by the css that i had modified. Jeff pointed me in the right direction in that img {max-width:100%;} was the issue.

To correct this i created the following css at the end to overide the img resizing for the ribbon.

.ms-cui-topBar2 img {max-width:none;}

Ran into the same issue after applying the responsive design and got this working in SharePoint 2013 :

.ms-cui-tabContainer img,#suiteBarButtons img {max-width:none !important;}